Al-rich Garnet Research Articles

Garnet Geochemistry of Reduced Skarn System: Implications for Fluid Evolution and Skarn Formation of the Zhuxiling W (Mo) Deposit, China

A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.

In situ LA-ICP-MS study of garnets in the Makeng Fe skarn deposit, eastern China: Fluctuating fluid flow, ore-forming conditions and implication for mineral exploration

Most garnets in Fe-Cu skarn deposits have simple petrographic textures, LREE-enriched geochemical patterns, are enriched in Fe, and are distinguished from garnets of typical W or W-Mo skarn deposits by their extremely low W, Sn, and Mo contents. However, little is known about the garnets in Fe skarn systems that have more complex textures and high concentrations of W, Sn, and Mo. Here we describe diverse textures and LA-ICP-MS data for garnets in the largest Fe skarn deposit (Makeng) in eastern China, and we discuss the dynamics of the mineralizing fluids and the significance of garnet for W-Mo exploration. Garnets from the Makeng deposit fall into four main types, corresponding to four-stage garnet growth based on their textural characteristics and trace element contents: Stage 1, Al-rich relatively homogeneous garnets (Adr51-55) with a portion (Adr17-48) and HREE-enriched patterns and weak negative Eu anomalies; Stage 2, Fe-rich garnet with LREE-enriched or flat patterns and weak negative or positive Eu anomalies, represented by oscillatory zoned rims (Adr50-90) with dodecahedral faces and oscillatory zones (cores Adr54-68, rims Adr65-80) with composite dodecahedral-trapezohedral faces; Stage 3, nearly pure andradite garnets with significantly LREE-enriched patterns and strong positive Eu anomalies, as represented by irregular zones (Adr70-99) in oscillatory zoned rims and relatively homogeneous garnets (Adr95-100) with extremely weak zoning; and Stage 4, Al-rich garnet with HREE-enriched patterns and weak negative or weak positive Eu anomalies, as represented by garnet veinlets (Adr45-77) with weak zoning. These four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model: a low flux responsible for the Al-rich garnet, a high flux responsible for the Fe-rich garnets, the highest flux responsible for the nearly pure andradite garnet, and a low flux responsible for the veins of Al-rich garnet. Relative to garnet in Fe-Cu skarn deposits, all garnet types in the Makeng Fe skarn deposit are rich in trace elements such as W, Sn, and Mo, especially the pure andradite garnet in stage 3, which has the highest contents of W, Sn, and Mo. These elemental enrichments are closely correlated with Fe, LREE/HREE, and fO2 signatures. The W and Mo signatures are identical to those of garnets in typical W or W-Mo skarn deposits, and they are useful geochemical indicators during exploration for W-Mo ore deposits.

Mineralogical and metasomatic evolution of the Jurassic Baoshan scheelite skarn deposit, Nanling, South China

The Baoshan scheelite deposit, located in the Nanling region, South China, is a skarn-type tungsten ore deposit. Orebodies occur in the contact zone between Carboniferous limestone and Baoshan granitic pluton. The pluton consists mostly of coarse-grained, porphyritic, and fine-grained biotite granites, which have zircon U-Pb ages of 172.4 ± 0.3 Ma, 166.6 ± 0.3 Ma, and 156.6 ± 1.4 Ma, respectively. These rocks have similar geochemical characteristics with high SiO2, total alkalis, high differentiation indexes and high Rb/Sr ratios of S-type granites formed by partial melting of upper continental crust. The porphyritic granites display the highest degree of fractionation, and are most closely associated with the formation of skarn ores.The skarns are zoned mineralogically outward from garnet, through clinopyroxene, to wollastonite, and overprinted by retrograde minerals. Three stages of skarnization are recognized: Early prograde, late prograde and retrograde. The early prograde stage is mainly composed of Al-rich garnet (Grs60–76) and Mg-rich clinopyroxene (Di41–62) with subordinate Mo-rich scheelite (Sch59–86), all of which are typically overgrown by late prograde assemblages of Fe-rich garnet (And67–95), Fe-rich clinopyroxene (Hd81–87), and Mo-depleted scheelite (Sch97–98). The retrograde stage is characterized by Al-rich garnet (Grs40–66) and pure scheelite (Sch100), accompanied by hydrous and sulfide minerals. Minor subcalcic garnet with magnetite sometimes occurs in the limited endoskarns. Fluid inclusions of the early prograde minerals homogenize at 390.5 to >550 °C, and one inclusion in wollastonite has a salinity of 13.0 equiv. wt% NaCl. Inclusions in minerals of the late prograde and retrograde stages have homogenization temperatures of 196.9–377.4 °C and 160.4–291.4 °C, respectively; their salinities 2.4–19.0 equiv. wt% NaCl and 1.4–11.6 equiv. wt% NaCl, respectively. Accordingly, the high-temperature and moderate- to high-salinity fluids in the early skarn stages are typical of magmatic fluids originating granites, whereas the lower temperature and variable salinity in the later stages suggest mixing with meteoric water.

Origin and Evolution of Oscillatory Zoned Garnet From Kasva Skarn, Northeast Tafresh, Iran

Within the central Urumieh-Dokhtar Magmatic Belt of northern Iran, the Kasva skarn deposit (KSD) formed by metasomatic alteration of Eocene intercalated carbonate and volcaniclastic sediments in response to Oligo-Miocene intrusion by granitoid porphyries. The KSD contains abundant oscillatory-zoned garnet crystals, which are characterized by isotropic cores of nearly pure andradite that are rimmed by anisotropic grossular–andradite (grandite). The Fe-rich andraditic cores are enriched in U and LREE, with positive Eu anomalies, whereas the Al-rich granditic rims are enriched in Ti, Nb, Zr, Hf, and HREE, without Eu anomalies. Variation in optical and chemical features in Fe- and Al-rich garnet are controlled by external factors such as (1) infiltration of compositionally distinct fluids, (2) incorporation of LREE and U at the {X} site in association with substitution of Fe^(3+) for Al^(3+) at [Y] within the crystal structure of andraditic garnet, and (3) substitution of Ti and HFSE for Al in granditic garnet.

Oscillatory zoning in skarn garnet: Implications for tungsten ore exploration

The Sangdong and Weondong deposits, Taebaeksan mineral district, South Korea, are tungsten skarn ore deposits. Andradite (Fe-rich) garnet is a characteristic mineral in these skarn deposits, and is characterized by notable trace element contents (W, Mo, Sn, and U) compared to grossular (Al-rich) garnet. We report characteristics of the trace elements including W, in the garnet structure from the vein-skarn system (outermost exoskarn) and identify a relationship of exploration significance between the trace element composition and W-ore. The chemical characteristics of the garnets were determined using Back-Scattered Electron (BSE) imagery, Electron Probe Micro-Analyzer (EPMA) point analyses and X-ray dot-mapping, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) point analyses and mapping. Garnets in the study area are mainly composed of grandite (grossular–andradite) garnets, and exhibit compositional variation as oscillatory zoning. The chondrite-normalized REE patterns of Al-rich garnets are HREE-enriched. In contrast, Fe-rich garnets are LREE-enriched. Tungsten is strongly fractionated into the Fe-rich garnets (average concentration: 618ppm), compared to the Al-rich rims (average concentration: 19ppm). In addition, other trace elements (Mo, Sn, and U) are also closely correlated with W and REE signatures. The Fe-rich garnets are attributed to rapid growth by infiltration metasomatism during disequilibrium, which caused the incorporation of these trace elements into the garnet structure that reflect the composition of the ore-forming fluid. Consequently, these results indicate that andradite is a useful indicator mineral in the exploration of tungsten ore deposits.

Metasomatic changes during periodic fluid flux recorded in grandite garnet from the Weondong W-skarn deposit, South Korea

The Weondong skarn deposit primarily consists of grandite (andradite-grossular) garnets, which exhibit poikilitic/intergrowth textures and compositional zoning in metasomatism. The garnets can be classified into three main types based on their textural characteristics and rare earth element (REE) concentrations: (1) exoskarn garnet near quartz porphyry with a poikilitic texture and oscillatory zoning (core: And31–50, rim: And54–63) (T1 garnet), (2) exoskarn garnet near quartz porphyry with intergrowths (core: And55–83, mantle: And55–59, rim: And77–97) and poikilitic textures (T2 garnet), and (3) vein-hosted garnet with reverse zoning (core: And70–96, rim: And31–36) and epitaxial growth on distinct cores (T3 garnet). The textural features of the garnets are considered to have been caused by periodic fluctuations. The chondrite-normalized REE patterns of the garnet types show that the Al-rich garnets are HREE-enriched and formed at equilibrium with slow growth rates according to the kinetics of the garnet's growth. In contrast, Fe-rich garnets are slightly LREE-enriched and HREE-depleted. In terms of the kinetics of the garnet's growth, the Fe-rich garnets grew rapidly from externally derived fluids during infiltration metasomatism. The enriched W contents in the grandite garnets also indicate the type of growth mechanism, the infiltration process, and a specific skarn environment (such as advective metasomatism). Furthermore, each textural type of garnet was influenced differently by fluid/rock interactions, the magnitude of the fluid flow and the fluid compositions in the skarn system. Consequently, periodic fluid fluctuation in the skarn system affected the morphology and composition of individual garnet grains. Thus, the geochemical features of the grandite garnet from the Weondong polymetallic deposit provide key information to understand the evolution of the skarn deposit and its metasomatic history.

Geochemistry of major and rare earth elements in garnet of the Kal-e Kafi skarn, Anarak Area, Central Iran: Constraints on processes in a hydrothermal system

Grossular-andradite (grandite) garnets, precipitated from hydrothermal solutions is associated with contact metamorphism in the Kal-e Kafi skarn show complex oscillatory chemical zonation. These skarn garnets preserve the records of the temporal evolution of contact metasomatism. According to microscopic studies and microprobe analysis profiles, the studied garnet has two distinct parts: the intermediate (granditic) composition birefringent core that its andradite content based on microprobe analysis varies between 0.68–0.7. This part is superimposed with more andraditic composition, and the isotropic rim which its andradite content regarding microprobe analysis ranges between 0.83–0.99. Garnets in the studied sample are small (0.5–2 mm in diameter) and show complex oscillatory zoning. Electron microprobe analyses of the oscillatory zoning in grandite garnet of the Kal-e Kafi area showed a fluctuation in chemical composition. The grandite garnets normally display core with intermediate composition with oscillatory Fe-rich zones at the rim. Detailed study of oscillatory zoning in grandite garnet from Kal-e Kafi area suggests that the garnet has developed during early metasomatism involving monzonite to monzodiorite granitoid body intrusion into the Anarak schist- marble interlayers. During this metasomatic event, Al, Fe, and Si in the fluid have reacted with Ca in carbonate rocks to form grandite garnet. The first step of garnet growth has been coeval with intrusion of the Kal-e Kafi granitoid into the Anarak schist- marble interlayers. In this period of garnet growth, change in fluid composition may cause the garnet to stop growing temporarily or keep growing but in a much slower rate allowing Al to precipitate rather than Fe. The next step consists of pervasive infiltration of Fe rich fluids and Fe rich grandite garnets formation as the rim of previously formed more Al rich garnets. Oscillatory zoning in the garnet probably reflects an oscillatory change in the fluid composition which may be internally and/or externally controlled. The rare earth elements study of these garnets revealed enrichment in light REEs (LREE) with a maximum at Pr and Nd and a negative to no Eu anomaly. This pattern is resulted from the uptake of REE out of hydrothermal fluids by growing crystals of calcsilicate minerals principally andradite with amounts of LREE controlled by the difference in ionic radius between Ca++ and REE3+ in garnet x site.

Garnets in porphyry–skarn systems: A LA–ICP–MS, fluid inclusion, and stable isotope study of garnets from the Hongniu–Hongshan copper deposit, Zhongdian area, NW Yunnan Province, China

The Late Cretaceous Hongniu–Hongshan porphyry–skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4–6.9‰) and low Dfluid (−142‰ to −100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22–57Grs78–43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12–39wt.% NaCl eq. and have liquid–vapor homogenization temperatures (Th) of 272–331°C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75–98Grs25–2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high ΣREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33–80wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465–591°C) than liquid–vapor homogenization temperatures (319–473°C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6km and reacted with carbonates at depths shallower than 2.0km. (3) Al-rich garnet veins (Adr14–60Grs86–40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids.

Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone

Garnet peridotite in oceanic subduction complexes has been reported only in two locations in the world. One of these examples occurs in the late Cretaceous to early Tertiary subduction complex in northern Dominican Republic. The garnet peridotite (wehrlite and olivine-bearing clinopyroxenite) occurs as large (≤ 4 m) boulders together with boulders of eclogites and serpentinites along a narrow (< 10 m) stream of the Rio Cuavas in the southern part of the Rio San Juan Complex. The peridotite is composed of garnet, diopsidic clinopyroxene (partially altered to magnesiohornblende), olivine (extensively altered to serpentine), Al-spinel and minor corundum; the latter two are mostly enclosed in garnet. Coarse-grained garnet also encloses small grains of early-formed garnet that contain Ca- and Al-rich cores and Mg-rich rims. The garnet peridotite contains low Cr, Ni, and Ir-group platinum group elements in bulk rock compared to primitive mantle values, low Mg (Mg#; 0.74–0.83) and NiO (< 0.1 wt.%) in olivine, and elevated concentrations of fluid-mobile elements (Sr, Pb and U) in clinopyroxene and bulk rock. Combined with the rare earth element data of bulk rocks and clinopyroxene, these data suggest that the peridotite originally solidified as a plagioclase-bearing cumulate of an arc melt at a shallow depth, < 35 km, in the mantle wedge. The cumulate was later dragged by mantle flow from the subarc mantle towards the subduction plane. Subsequent downward movement along the subduction plane resulted in the crystallization of corundum and Ca- and Al-rich garnet at the expense of plagioclase, at a depth of ~ 50 km. The garnet peridotite continued to be subducted to a depth of ~ 120 km, causing a progressive increase in pressure and temperature that resulted in the crystallization of Mg-rich garnet. The garnet periodotite was then exhumed in the serpentinite subduction channel. These events likely took place during the very early stages of the subduction system, where a strong mantle corner flow was likely produced by poor lubrication along the interface between the subducting plate and the overlying Caribbean Plate.

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe 3+) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids. Garnets from the Crown Jewel deposit range from Adr 30Grs 70 to almost pure andradite (Adr >99). Fe-rich garnets (Adr >90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE 3+ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr >90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE 3+ following a coupled, YAG-type substitution mechanism ( [ X 2 + ] - 1 VIII [ REE 3 + ] + 1 VIII [ Si 4 + ] - 1 IV [ Z 3 + ] + 1 IV ) , whereas Eu 2+ substitutes for X 2+ cations. Thermodynamic data (e.g., H mixing) in grossular–andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions. Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/ R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/ R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/ R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.

Magmatic modification and metasomatism of the subcontinental mantle beneath the Vitim volcanic field (East Siberia): evidence from trace element data on pyroxenite and peridotite xenoliths from Miocene picrobasalt

The genesis of several groups of pyroxenite xenoliths from Miocene picrobasalt of the Vitim volcanic field are considered on the basis of petrology and mineral chemical data, including trace element analyses of minerals by ion probe. The pyroxenites and related xenoliths can be classified into three groups consisting of 10 sets of xenoliths: Group I (Cr-diopside series) is made up by three subdivisions; (Ia) common Cr-diopside garnet and spinel websterites, (Ib) Al-poor Cr-diopside websterites, (Ic) modally metasomatized Cr-diopside lherzolites. Group II (Al-augite series) consists of four further xenolith types; (IIa) Cr-rich and (IIb) Cr-poor megacrystic pyroxenite assemblages, (IIc) Al-rich garnet clinopyroxenites, and (IId) amphibole and/or phlogopite-bearing rocks and associated orthopyroxenites. Group III consists of xenoliths all characterized by Ca-rich clinopyroxene (garnet clinopyroxenite and gabbros, garnet granulites and spinel websterites) thought to have a similar depth of origin near the crust–mantle boundary. Trace element data and textural criteria allow the distinction of three melt types operating in the mantle: (1) Cr-rich melt migrating along small fractures and forming Cr-diopside pyroxenite veins as the result of percolation fractionation; (2) picrobasaltic to alkaline basaltic melt fractionating in large hydraulic fractures or magma chambers and producing host volcanics, clinopyroxene megacrysts and megacrystic pyroxenites (Groups IIa–IIb); and (3) high field strength element (HFSE)-enriched smaller volume melts, which formed the hydrous mineral-bearing xenoliths of Groups I and II (Ic and IId). The megacrystic pyroxenites (IIa–IIb) were crystallized in magma chambers or large fractures at various depths during multistage polybaric fractionation. The high-temperature Cr-rich websterites (Group IIa) correspond to the deepest level at near 30 kbar. The garnet websterites and megacrystic clinopyroxenites crystallized at intermediate levels, whereas the most differentiated megacrystic clinopyroxenites with ilmenite and phlogopite inclusions are from the shallowest levels at 15–11 kbar. The HFSE-rich melt resulted in amphibole- and phlogopite-rich assemblages crystallized in veins and interstitially in peridotites, and the development of ilmenite–phlogopite-bearing orthopyroxenites in a reaction zone between these. Pyroxenites of Groups I and II represent samples of an intensely veined sub-Vitim lithosphere: the preservation of textural and chemical disequilibrium indicates that the enrichment immediately preceded xenolith sampling and is related to the upward and outward migration of the asthenosphere–lithosphere boundary beneath the developing Baikal Rift. The formation of lower pressure xenoliths (Group III) is related to both of the first two melt types mentioned above. Spinel websterites of Group III are genetically related to the Group Ia melt crystallized near the crust–mantle boundary, and the mafic garnet granulites of Group III to intermediate fractionation products (gabbro-norites) of similar melts. Garnet gabbros are associated with the most fractionated megacrystic pyroxenites.

Activation of microglia and expression of heat shock protein during kainic acid-induced hippocampal seizure in the rat

The Late Cretaceous Hongniu–Hongshan porphyry–skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4–6.9‰) and low Dfluid (−142‰ to −100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22–57Grs78–43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12–39 wt.% NaCl eq. and have liquid–vapor homogenization temperatures (Th) of 272–331 °C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75–98Grs25–2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high ΣREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33–80 wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465–591 °C) than liquid–vapor homogenization temperatures (319–473 °C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6 km and reacted with carbonates at depths shallower than 2.0 km. (3) Al-rich garnet veins (Adr14–60Grs86–40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids.

Notice of three specimens of garnet

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe3+) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids.Garnets from the Crown Jewel deposit range from Adr30Grs70 to almost pure andradite (Adr>99). Fe-rich garnets (Adr>90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE3+ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr>90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE3+ following a coupled, YAG-type substitution mechanism ([X2+]-1VIII[REE3+]+1VIII[Si4+]-1IV[Z3+]+1IV), whereas Eu2+ substitutes for X2+ cations. Thermodynamic data (e.g., Hmixing) in grossular–andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions.Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.